Single vibronic level (SVL) fluorescence spectra following excitation into the 0 00 band and many sequence and cross-sequence bands in the Ã1A'- X∼1A' system of styrene have been recorded and assigned. The technique is shown to be very powerful in obtaining accurate energy levels for low-wavenumber vibrations, particularly in the X∼ state, where it is capable of giving information regarding vibrational levels inaccessible by conventional infrared or Raman spectroscopy. Interpretation of the spectra leads to many new assignments in the absorption spectrum and to an accurate knowledge of many vibrational energy levels. An important reassignment is that of the 41 0142 01 band previously assigned to 40 02 ( ν40, ν41, and ν42 are all a″ vibrations). The two most important pieces of information which derive from analysis of the SVL spectra are (a) the first five vibrational levels of ν42, the C(1)- C( α) torsional vibration, in the X∼ state, each accurate to about ±0.2 cm -1, and (b) the fact that the normal coordinates of ν42 and ν41, an out-of-plane substituent vibration, are very heavily mixed in the Ã, relative to the X∼, state—an extreme case of the Duschinsky effect. As a result of analysis of the SVL spectra, analysis of the absorption spectrum is now so complete that we can be confident that the Hui and Rice proposal that the CH 2 group of the substituent is perpendicular to the plane of the rest of the molecule in the Ã state has no evidence to support it.